The present invention relates generally to ultraviolet (UV) curable coating compositions and their use as basecoats on metallized molded plastic articles such as lamp reflectors.
As further background, metallized plastic molded articles are multi-layer structures which include a molded substrate, a basecoat on the substrate, and a metal film adhered to the basecoat. Further, metallized plastic molded articles may include one or more additional coatings on top of the metal film, for example, they may simply include a topcoat or may include an interlayer and a topcoat.
The basecoat is a highly important component of metallized molded plastic articles. The basecoat must adhere well to the plastic substrate and to the metal film. Additionally, the basecoat must be of a uniform thickness across the coated area of the substrate in order to provide a smooth, lusterous appearance to the metallized article and, in the case of metallized articles having exacting optical standards, e.g. reflectors, to preserve the intended optics of the final device. A problem which has been encountered in the prior art is that basecoats have not possessed good edge-coating or leveling properties. Rather, basecoats used in the past have tended to draw or creep away from edges of the substrate thus leaving only a thin coating or no coating near the substrate edges, or have tended to gather at edges and cure to form an undesired ridge or raised portion. In the case of automobile lamp reflectors, this problem has hampered the production of quality, substantially smooth-surfaced reflectors which are used in conjunction with lenses which contain optical structure to achieve appropriate distribution of light.
Moreover, a trend exists in the automobile industry to eliminate optical structures from headlamp lenses and rather build them into the associated reflectors. Such reflectors having built-in optical structures include, for example, fluted and/or faceted reflectors (see for instance U.S. Pat. Nos. 4,704,661 and 5,034,867). Fluted reflectors include a plurality of convex flutes which introduce alternating grooves and ridges or humps within the reflector surface. Faceted reflectors have a plurality of discreet reflective surfaces which introduce grooves and/or edges between the discreet surfaces. Basecoats having poor edge-coating and leveling properties present special problems with such optical reflectors because thinning or gathering on or between their structural features interferes with the intended optics.
Basecoats on metallized plastic molded articles, especially in the case of lamp reflectors, must also exhibit superior thermal stability. Generally, in testing, such basecoats must form stable substrates for the metal film, and the resulting reflectors must exhibit no change in appearance (generally no thermal blistering, cracking or hazing) when subjected to a minimum of 350.degree. F. for two hours. In the case of lamp reflectors formed from bulk molding compound ("BMC"--glass-reinforced unsaturated polyester resin compositions typically containing up to about 30 wt. % or more glass fibers, e.g. 10%-30%), achieving these aims has been difficult since coatings which are thermally stable have not adhered at all well to the BMC surface.
A need therefore exists for UV curable compositions which are useful as basecoats for metallized molded plastic articles and which exhibit superior flow properties with respect to edge-coating and leveling, and which form cured coatings which adhere well and exhibit thermal stability. The present invention addresses these needs.